In recent years, it is generally known that, in a system (hereinafter referred to as a “network boot system”) that boots an operating system (hereinafter referred to as an “OS”) through a network, in order to reduce load on a network boot server, a read-only cache (hereinafter referred to as a “read cache mechanism”) is provided in a part of a physical storage device on a client terminal side.
In Patent Document 1, a network boot system is disclosed in which a read cache mechanism is provided that can reduce network access to a network boot server by caching a virtual disk on the network boot server in a local disk of a PC (client terminal) that is used by an end user in the network boot system. The virtual disk is generation-managed using revise information (hereinafter referred to as “revision”) and the revision is updated along with revision of the virtual disk such as installation of an updated program and a configuration change. The read cache mechanism has a cache management table corresponding to the revision of the virtual disk in each terminal and presence or absence of cache data is judged using the management table.
Further, the above-described network boot system further has a modified region map that indicates a correspondence relationship between a region of the virtual disk and a cache region. When the virtual disk is newly revised, the revision of the virtual disk is advanced from an old to a new revision and the modified region map indicating a modified cache region is applied to the cache management table. By using the management table, the read cache mechanism discards cache data corresponding to data that has been modified in the virtual disk and re-caches the data, and holds cache data corresponding to data that has not been modified as cache as is. When the virtual disk is newly modified and the revision is advanced in this way, the read cache mechanism can keep and effectively utilize a large portion of the cache. In such a network boot system, even when a large number of client terminals are booted all at once, for a second or subsequent booting, there is almost no access to the network and the client terminals can be booted at high speeds.